1. Field of the Invention
The invention concerns generally a method for the manufacture of a superconductor and more particularly, the manufacture of one having a superconductive intermetallic compound.
2. Description of the Prior Art
Superconductive intermetallic compounds consisting of two elements, of the type A.sub.3 B, e.g., Nb.sub.3 Sn or V.sub.3 Ga, which have an A-15 crystal structure, exhibit very good superconduction properties and are distinguished particularly by a high critical magnetic field, a high transition temperature and a high critical current density. They are therefore highly suitable for use as superconductor coils for generating strong magnetic fields such as those needed for research purposes. Other applications are in superconducting magnets for the suspension guidance of magnetic suspension railroads or in windings of electric machines.
Several methods for manufacturing these superconductors in the form of long wires or ribbons are known. They are particularly employed in the manufacture of so-called multi-core conductors having wires, particularly of Nb.sub.3 Sn and V.sub.3 Ga, arranged in a normal-conducting matrix. Generally in these methods a ductile element of the compound to be manufactured in wire form, e.g., a niobium or a vanadium wire, is surrounded by a jacket of an alloy comprising a ductile carrier metal and the other elements of the compound, for instance, a copper-tin alloy or a copper-gallium alloy. In particular, a multiplicity of such wires are embedded in a matrix of the alloy. The structure so obtained is then subjected to a cross section-reducing process. This results in a long wire such as is required for coils wherein the diameter of the niobium or vanadium wire is reduced to a value on the order ot about 30 to 50 .mu.m or even less. The latter is desirable in view of the superconduction properties of the conductor. One further seeks to obtain through the cross section-reducing processing the best possible metallurgical bond between the wire and the surrounding matrix material of the alloy, without the occurrence of reactions that lead to an embrittlement of the conductor. After the cross section-reducing processing, the structure is subjected to a heat treatment such that the desired compound is formed through reaction of the wire material, with the other element contained in the surrounding matrix-in one example the tin or gallium. The element contained in the matrix diffuses into the wire material and reacts with the latter, forming a layer of the desired compound (German Offenlegungsschriften Nos. 2,044,660; 2,052,323 and 2,105,828).
These known methods are not satisfactory for a number of reasons. First, the diffusion process cannot be directed in such a manner that all the gallium or tin present in the matrix for forming the intermetallic compound is consumed. It is therefore not possible to build up V.sub.3 Ga or Nb.sub.3 Sn layers of any desired thickness. The diffusion of gallium or tin in the direction toward the vanadium or niobium cores comes to a standstill when the activity of the elements gallium and tin in the copper matrix is equal to their activity in the produced intermetallic compounds V.sub.3 Ga or Nb.sub.3 Sn. In other words no further V.sub.3 Ga or Nb.sub.3 Sn is formed if the concentration of the gallium or tin in the copper matrix has dropped to a certain value due to the diffusion of gallium or tin into the cores. For instance, if one diffuses gallium into vanadium cores from a copper-gallium matrix with 18 atom-percent of gallium at a temperature of 700.degree. C., the equilibrium condition mentioned is reached when the gallium content of the matrix has dropped to about 12 atom-percent. This means that only about 38% of the gallium available in the matrix has been converted to V.sub.3 Ga.
The thickness of the Nb.sub.3 Sn or V.sub.3 Ga layers formed in a multi-core conductor is therefore dependent not only on the annealing time, the annealing temperature and the composition of the copper-gallium or copper-tin alloy, but also by the total amount of tin or gallium available for each core, i.e., on the volume of the part of the matrix available for each individual core.
In order to achieve a high effective critical current density, i.e., a high critical current density referred to the entire conductor cross section, however, layers thick as possible of the intermetallic compound to be prepared are required. Employing the prior art methods this can only be achieved by making the ratio of the matrix component to the core component of the total cross section area of the conductor such that the growth of the layer is not limited by the available gallium or tin content in the alloy. In other words, a core spacing as large as possible is necessary. In multi-core conductors of given cross section, however, this requirement can only be met either by drawing the cores, if the number of cores is fixed, particularly thin in the cross section-reducing processing, or by reducing the number of the cores, if the core cross section is fixed. Either solution is not very satisfactory. On the one hand the drawing of the cores to form very thin filaments presents considerable difficulties and is quite expensive. On the other hand, if the number of cores is reduced, the effective current density is decreased and may only be offset generally by the thicker diffusion layers that may possibly be obtained. Finally, an arbitrary increase of the core spacings is not possible for technical reasons inherent in the deformation process. This is due to the fact that if one wishes to draw a larger number of vanadium or niobium cores uniformly thin in such a manner that their cross sections remain equal, the core spacing must not be too great.
A further difficulty with the known methods is that the matrix material containing the embedded cores and consisting of the carrier metal and the other elements of the compound to be manufactured is relatively hard to deform, particularly with higher concentrations of these elements. These matrix materials have the property that they harden very quickly in a cross section-reducing, cold-working process and become very difficult to deform further. With these methods it is therefore necessary to subject the conductor structure consisting of the cores and the matrix material, after relatively small deformation steps to an intermediate anneal for recuperating and recrystallizing the matrix structure. Although these heat treatments can be performed at temperatures and with annealing times at which the superconductive compound to be manufactured will not form, they are very time-consuming because of the required frequency. This increasing degradation of the deformability of the matrix materials with increasing content of the remaining elements of the compound to be manufactured limits their concentration and thus the thickness of the intermetallic compound layer. Further, with increasing concentration of these elements the melting point of the matrix material decreases. At very high concentrations this leads to problems in the heat process necessary for forming the intermetallic compounds. Furthermore, these elements can form undesired intermetallic phases with the carrier metal, if their concentration is too high.
There are known methods for avoiding the aforementioned, repeated intermediate anneals. In these methods, one or several cores of the ductile element are embedded in a ductile matrix material, e.g., copper, silver or nickel, which itself contains no element of the compound to be produced, or only very small amounts of such an element. The structure consisting of the cores and this matrix material is then processed without any intermediate anneal into a thin wire by a cross section-reducing process, as for example the cold-working process. After the last cross section-reducing step, the remaining elements of the compound to be produced, i.e., tin in the case of Nb.sub.3 Sn, are then applied to the matrix material. This is accomplished by immersing the wire briefly into a tin melt, so that a thin tin layer is formed on the matrix material, or by evaporating a tin layer onto the matrix material. Subsequently a heat treatment is then performed, in which the elements applied to the matrix material, are first diffused into and through the matrix material and then form the desired superconductive compound through reaction with the cores ("Applied Physics Letters" 20 (1972 ), pages 443 to 445; German Offenlegungsschrift No. 2,205,308).
However, in this method only relatively small amounts of the element can be applied to the matrix. If larger amounts of tin are applied, undesirable brittle intermediate phases of copper and the element can readily form at the temperature required for the diffusion of the element into the copper matrix. Further, if too much of the element is applied, the element itself or a surface area of the matrix can melt at the temperatures necessary for the diffusion of the element and can then easily drip off or run off from the matrix surface. The result is that only a limited amount of the lower-melting element is therefore available for the formation of the desired intermetallic compound.
German Offenlegungsschrift No. 2,205,308 teaches an expensive, multi-step process for converting all the niobium contained within a copper matrix into Nb.sub.3 Sn. The process calls for repeating the individual process steps of coating the matrix with tin and reacting the tin contained in the matrix with the niobium cores.
The continuous method for the manufacture of multi-core conductors of Nb.sub.3 Sn described in the German Offenlegungsschrift No. 2,205,308 is a method wherein the conductor structure in wire form, consisting of a copper matrix and embedded niobium cores, is continuously conducted through an oven, in which several vessels with melted tin are arranged side by side. The conductor structure runs successively through the part of the oven interior situated above these vessels and then exits from the oven. The tin melt, through whose corresponding vapor space the conductor structure first runs is at a temperature of 1500.degree. C. The other tin melts, are at a temperature of 1000.degree. C. The conductor itself is held by the oven at a temperature of 850.degree. C. Apparently, the tin vapor pressure in the vapor space over the first tin melt is so high that the transfer or deposition rate of the tin exceeds the solid diffusion rate of the tin into the copper matrix. Thus, a tin concentration gradient builds up quickly across the wire radius. The conductor structure is held above the tin melt of the higher temperature until sufficient tin, determined by the desired mean matrix composition, is applied. When the conductor structure next passes over the lower temperature melts, the tin vapor pressure is just high enough that the tin supply rate is reduced to a value which is equal to that at which the tin diffuses through the copper matrix and arrives at the surfaces or the niobium cores. The solid diffusion itself takes place at the temperature of 850.degree. C. This temperature is chosen in order to prevent the tin from evaporating from the matrix and to prevent the matrix from melting. This method, however, is also extremely expensive because of the three different temperatures required for the tin melts and the conductor structure and because the conductor structure itself must be held accurately positioned during the relatively laborious process.
The following therefore are some of the objects of this invention:
To provide a process for manufacturing a superconductor wherein the layer thickness of the superconductive, intermetallic compound is not limited by the process;
to employ a ductile matrix material that can be deformed cold without intermediate anneals;
to form the finished product employing but one heat treatment temperature and thereby simplify the procedure substantially;
to provide a process for producing a superconductor having a superconductive intermetallic compound layer comprising at least two elements which conductor can have various forms;
to provide a process whereby the thickness of the layers of superconductive intermetallic compounds can be controlled by varying the individual process parameters; and
to provide an apparatus for implementing the process of the invention which does not inhibit access of the metal vapor to the conductor surface.